-- ==============================================================================
-- RailwaySystem 数据库 V2 增量脚本: 020_add_checkin_student_id.sql
-- 目标: 为 train_student_checkin 增加 student_id (VARCHAR(32))
--       把学员维度从 enrollment 关系下沉为考勤行本身的冗余列，按学员检索考勤时
--       无需再 join train_enrollment，同时保留 enrollment_id 作为业务唯一键。
-- 适用数据库: KingbaseES V009R003C018 / PostgreSQL (schema: public)
-- 幂等性: 通过 information_schema 判断列是否已存在，可重复执行
-- 关联变更: 004/006 基线同步、010 校验规则调整、Mapper/Domain/DTO/Service 同步
-- ==============================================================================

BEGIN;

-- 步骤 1: 新增列 (可为空，先加列再回填，避免已有数据阻塞)
DO $$
BEGIN
    IF NOT EXISTS (
        SELECT 1 FROM information_schema.columns
        WHERE table_schema = 'public'
          AND table_name = 'train_student_checkin'
          AND column_name = 'student_id'
    ) THEN
        ALTER TABLE train_student_checkin ADD COLUMN student_id VARCHAR(32);
    END IF;
END $$;

COMMENT ON COLUMN train_student_checkin.student_id IS '学员ID (冗余自 train_enrollment.student_id，随考勤行落库)';

-- 步骤 2: 以参训档案为准回填历史数据
UPDATE train_student_checkin a
   SET student_id = e.student_id
  FROM train_enrollment e
 WHERE e.enrollment_id = a.enrollment_id
   AND (a.student_id IS NULL OR a.student_id <> e.student_id);

-- 步骤 3: 按学员维度检索的索引
CREATE INDEX IF NOT EXISTS idx_checkin_student ON train_student_checkin (student_id);

-- 步骤 4: 回填结果校验 (存在无参训档案的脏数据时仅告警，不中断发布)
DO $$
DECLARE
    v_null_cnt INT;
BEGIN
    SELECT count(*) INTO v_null_cnt
      FROM train_student_checkin
     WHERE student_id IS NULL;
    IF v_null_cnt > 0 THEN
        RAISE NOTICE '>>> [CheckinStudentId-WARN] 仍有 % 行 student_id 为空，请核对 enrollment_id 是否有效', v_null_cnt;
    ELSE
        RAISE NOTICE '>>> [CheckinStudentId-PASS] train_student_checkin.student_id 已全量回填';
    END IF;
END $$;

COMMIT;